Antiflex-cracking agents in rubber



Patented Jam 8, 1952 UNITED STATES PATENT OFFICE No Drawing. Application July 1, 1048, Serial No. 36,800

Claim.

This invention relates to the stabilization of natural rubber by the incorporation therein of a material which prevents or inhibits cracking of the rubber on flexing. The flex-cracking inhibitor employed is a cresylic acid alkylated with introduction of at least about one, and generally no more than about two, hydrocarbon substituents of four to twelve carbon atoms for each molecule of the acid. It has been found that the alkyiated cresylic acids do not discolor rubber as do the most commonly used commercial flex-cracking inhibitors. These badly discolor light-colored rubbers, such as the white sidewalls of tires and other rubbers heavily loaded with white,;pigment.

Cresylic acids are I obtained from petroleum sources and also from coal and wood derivatives. as by the treatment of coaland wood-tar distillates. They are available in large quantity and are relatively inexpensive. The term "cresylic acid is used to designate a mixture of phenolic materials, particularly cresols, xylenols. trimethylphenols, tetramethylphenols, ethylphenols and methylethylphenols, and also some higher homologues. The higher homologues include at most no more than a trace of any propyl or higher alkylphenol.

One method of obtaining petroleum cresylic acid is to extract a cracked petroleum distillate with aqueous caustic soda and thus obtain the salts of the mixture of acids which go to make up the cresylic acid. The resultant solution containing some dissolved oil separates from the distillate by settling. This oil is removed by steam distillation, and the residual solution is acidified to free the cresylic acid which. on standing, rises to the top of the water solution. The acid is then usually recovered by distillation in a vacuum, and it is customary to separate the distillate into fractions of different boiling ranges. These may be further purified if desired. The total acid includes cresols, ethyl-, dimethyl-, trimeihyl-. and tetramethylphenols. It contains no more than a trace. if any. of a phenol allqrlated with a group which contains three or more carbon atoms.

The mixture oi acids in the lower boiling fraction of a. cresylic acid. whether it be derived from petroleum. coal or wood. is of the same general composition. This is true of the cresylic acids which boil from 190 to 250 C. at atmospheric pressure. The alkylation derivatives of such acids form the preferred stabilisers of this invention.

The compositions of the higher boiling fractions are quite different. The higher cuts of coal-tar cresylic acids-tor example. those boiling above about 250 C.-contain fused ring compounds. such as naphthols. etc. The alhlation derivatives of these higher boiling coal-tar cresylic acid fractions are not as desirable for use in rubber as the alnlation derivatives of the higher cuts of petroleum cresylic acids. The latter are also preferred to the alkylation derivatives of the higher cuts of the wood-tar cresylic acids.

The boiling range of the alkylated cresylic acids varies. In general. it may be said that these compounds have an initial boiling point of at least C. at 10 mm. pressure. If a butylenefor example. isobutylene, is used for the alkylation, the reaction product will begin to boil at about 100 C. under 10 mm. pressure. If diisobutyiene is used the initial boiling point will be in the neighborhood of about C. at 10 mm. pressure.

Thus, the stabilizers of this invention are allnvlated cresylic acids. Although a, higher boiling fraction of a etroleum cresylic acid may be used for the alkylation. or a higher cut of an alkylated petroleum cresylic acid may be used. and is included within the scope of this invention. the invention relates more particularly to the use of alkyiated lower cuts of those cresylic acids which boil at not over 250' C.. and these preferred inhibitors may be derived from any a]- kylated cresylic acid whether of petroleum, coal or wood origin. Of those acids which boil at 250 C. and below, the lower boiling constituents are more easily alkylated than the higher boiling, and because of the ease with which they can be prepared they are generally preferred to the more diflicultly alkylated, higher boiling constituents. whatever fraction of the cresylic acids is employed. and whatever the process of alkylation, it results in introduction of substantially one to two alkyl groups per molecule.

In certain of the following examples the boiling range of the unaikylated starting material overlaps the boiling range of the alkylated product. It does not follow that the alkylated product contains any considerable percentage of unalkylated material, because the lower boiling alkylated cresylic acids boil within the range of the higher boiling unalkylated cresylic acids. Actually. unalkylated cresylic acid has little stabilizing action on natural rubbenwhereas the alkylated product is a, very effective stabilizer. Moreover. the unalkylated material is more volatile and has an oiiensive odor not present in the alkylated product. In the claims the bofling range attributed to the alkylated product has been selected to avoid overlapping, because the percentage products in the range of the overlap is inappreciable and it would be impractical commercially to use a fraction of such limited boiling range.

Alkyiation may be eil'ected with isobutylene, diisobutylene, triisobutylene, dipropylene. tripropylene, tetrapropylene, mixed heptenes, mixed actenes, mixed nonenes, mixed decenes, undecenes, or other olefin or a mixture of olefins, or by a medal-Crafts reaction using an alkyl halide of three or more carbon atoms. such as a chloroor bromo-derivative of n-butane. isobutane, any pentane. hexane. heptane. octane, nonane, decane. etc.. or a mixture thereof. In general. the stabilizers will contain no alkyl group higher than dodecyl. Any suitable method. alkylation may be employed, such as the use of an aliphatic alcohol corresponding to the olenns or alkyl halides mentioned above. etc.

The following examples illustrate the preparation of the alkylation derivatives.

EXAIMPLEI This illustrates the alkylation of a higher-boiling cresylic acid. boiling at not over 250 C.

One hundred grams of petroleum cresylic acid (B. P. 221-245" C. at atmospheric pressure or 08 to 118 C. at mm.) mixed with 50 g. toluene as diluent, and as catalyst, 40 g. of 1:1 mixture of cone. sulfuric acid and anhydrous glyceririe, was maintained at 70 to 75 C. ior three hours while a stream of isobutylene was passed through it. The upper layer was then decanted from the catalyst solution. washed with water and with sodium carbonate solution, and then distilled to yield 127 g. of light yellow liquid of n "=l.5198 and B. P. 101-157 C. at 10 mm.

The above preparation was fractionated and the following two fractions alkylated further:

One hundred grams of the fraction boiling 100- 120 at 10 mm. was stirred with g. aluminum chloride while 190 g. of tert-butyl chloride was run in with stirring over six hours at 3&44' C. The reaction mixture was poured into water. the product extracted with petroleuum ether and washed with successive portions of sodium bicarbonate solution, sodium carbonate solution and water. After removal of solvent. the product was distilled, the portion boiling from 103 at 10 mm. to 210 at 4 mm. being blended with the final product from the second fraction.

One hundred grams of the fraction boiling between 120-156 C. at 10 mm. was mixed with 20 g. of aluminum chloride and stirred at 87-45 while 200 g. of tert-butyl chloride was run in over three hours. After an additional five hours of stirring the reaction mixture was poured into water, extracted with petroleum ether and washed with successive portions of sodium bicarbonate solution, sodium carbonate solution and water. After drying over anhydrous sodium sulfate and removal of solvent the product was distilled. the material boiling from 110' C. at 10 mm. to 198 at 3 mm., being combined with the alkylated lower fraction described in the previous paragraph.

The resulting blend boiled at 100 C. at 10 mm.

4 to 210' C. at 4 mm. and was used in natural rubher as described below.

EXAIWPLEZ" The following example illustrates the octylation of a lower-boiling cresylic acid.

One hundred grams of petroleum cresylic acid (3. P. 195-228 C. at atmospheric pressure or 82 to 106' C. at 10 mm.) and 10 grams of boron fluoride etherate were stirred at 40-75 C. while 200 gr of diisobutylene was dropped in over one hour.

Stirring was continued for two hours and the mixture allowed to stand 15 hours at 55-60 C.. after which it was washed with sodium carbonate solution, then with aqueous sodium hydroxide, and lastly with water. The octylcresylic acid thus obtained was diluted with ether, dried over anhydrous potassium carbonate and distilled. B. P. 140' C. at 10 mm. to 167 C. at 3.5 mm. Yield 93 a.

Carbon and hydrogen analysis:

Per Cent 0 Per OentH Calculated for monooctyl cresylic acid... 412. 0 ii. 1

Calmlated fordicctyl wesylic acid.....-.. 83.0 121 Found 81.71 11.56

EXAMPLE 3 The cresylic acid used in this preparation is a petroleum cresylic acid fraction with a boiling range of 92 to 121 C. at 10 mm. or 210-250 C.

' at 760 mm. It is substantially the some out as used in Example 1.

One hundred grams of the acid were mixed with 200 grams of diisobutylene. Forty grams of a 1:1 mixture of sulfuric acid and anhydrous glycerol was added as the catalyst at 5 to 10 C. The reaction mixture was stirred for four hours at 5 to 10 C.. and then it was washed to remove the catalyst. After drying over anhydrous sodium sulfate. the octylcresylic acid thus obtained was distilled. Thirty-seven grams of product was taken at 122 to 166! C. at 10 mm. Such a product r was used in the following tests.

The several inhibitors were incorporated in a white rubber stock such as the following:

Parts by weight The compounds were cured and tested against a blank which was similarly compounded. but contained no inhibitor. The several samples were subjected to a flexing test in which -inch dumbbell strips of 0.100 gauge were flexed from 0 to 75 per cent elongation until all strips were broken. The strips were then examined under a ma nifying lens and the number of cracks estimated. Totaling the number of hours required for the strips of each different stock to break, the flex life was obtained. From the total number of cracks and the total number of hours run, the average rate of crack formation was calculated. The results of the tests on the various stocks are recorded in the following table. The inhibitors are identified simply by reference to the examples which dwflbe how they may be prepared.

Flu Life. lflours These stocks were exposed to weathering tests. Stocks containing inhibitors such as obtained by the processes of Examples 1 and 3 were weathered 8 weeks in Florida. The test stocks and blanks colored The blank showed more surface cracks than the test stock.

EXAMPLE 4 A petroleum cresyiic acid was alkylated to produce the product known herein as Example 4. Fractions of the product are identiiied herein as Examples 4A, 43 had 4C. Their various boiling ranges are given in the table below.

Sixty pounds oi petroleum cresylic acid (3. P. 195 to 225 C. at atmospheric pressure: 82 to 102 C. at 10 mm. pressure) and 50 pounds 01' dilsobutylene were stirred at -70 C. while 4 pounds of boron fluoride etherate was run in during the course of one hour. After two hours continued stirring at 05-70 C. the mixture was allowed to stand over night. It was then washed with water. and aqueous solution or sodium bicarbonate and sodium sulilte. and again with water. The product was then distilled. first under atmospheric pressure. then under reduced pressure. The material distilling at 140-173 C. at mm. was taken as the product of Example 4. motions obtained are identified in the following table:

BoiiingBs-nge Example lD-ln' Cm mm. 140-192 (1/10 mm. ISO-1M 0.1" mm. lot-173 CJlli mm.

Exposure to sun lamp liengihoiCure Btabiliwr filliiuutes I Minutes 00 Minutes Hillinutes Verysiightly White Whitecreamy.

dc Do. '-do Do. Slightly .-do Do.

maboveisevidencethatthealkylated cresylic acids donotdeepentheoolcroithscm-edrubber compound. These stabilizers therefore belong to the non-discolorlng type.

Samples of the cured rubber were tested also against the blank containing no stabilizer. in

order to determine the eilect oi the various stabilizers on the flex life of the rubber. Bight s rli swereussdineachtest,2stripsbeingcm'ed at 200' l. for minutes, 40 minutes. 80 minutes and so minutes respectively. The results are recorded in the following tables:

Per can! The other stabilizers were similarly tested against a blank with the following results:

These test results show that the stabilizers definitely inhibit flex-cracking.

EXAMPLES ,A petroleum cresylic acid was nonylated and 0 used as a stabilizer in rubber with very satisfactory results. Certain of the tests made on this Product are recorded below.

The stabilizer was prepared from a petroleum cresylio acid having a boiling range of 193 to 220 C. at atmospheric pressure. (78 to 98 C. at

10 mm.). Three hundred grams of the acid and 300 grams oi a mixture of nonenes (obtained by polymerisation o! propylene) were treated with 30 grams of boron trifluoride-ether complex. The

temperature of the reaction mixture rose to 35 C.

The reaction mixture was heated at 60-65" C. for two hours. Alter cooling it was washed with water. with a 2 per cent sodium suliite-2 per 'cent sodium bicarbonate mixture, and finally with water. The crude yield of 010 grams was 235 grams (n,,"=l.5l45) distilled under reduced pressure. The forefractton up to 140' C. at 10 mm. contained 365 grams of material. The product was taken at 140 c. at 10 mm. to 205 C; at 3.5 mm., and weighed The i'oreiraction was treated with 50 grains of anhydrous aluminum chloride. Addition of the aluminum chloride caused the temperature to rise to C. and then the reaction mixture was as heated at loo- C. for two hours. After washing with water. with 2 per cent sodium blcarbonate and 2 per cent sodium suliite. and water. the product was distilled. The foreiraction (up to C./l0 mm.) weighed 105 grams.

in The product was taken at 140 C. at 10 mm. to

250 C. at 2 mm. and weighed grams (n =1.5150). This yield was 27.5 per cent of the total reactant weight. Hence, the combined yield was 00.! 'per cent oi the total reactant 7 This nonylated cresylic acid wasoompounded with rubber according to the following formula:

Parts by weight Pale crepe rubber 100.0 Zinc oxide 85.0 Titanium dioxide 10.0 Insoluble sulfur 3.0 Ultramarine blue 0.2 Btearic acid 1.2 Dibutyl ammonium oleate 0.5 Benmthiazolyl disulilde 0.4 Sunproof wax 2.0 Stabilizer 1.0

The material compounded according to the above formula was tested for comparison with a blank similarly compounded but omitting the stabilizer. Cures were made at 280 F. for 20, 40, 60 and 80 minutes. The two vulcanizates were tested with the following results:

NORMAL TENSILE PROPERTIES TENBILE raogs g'rms AFTER AGING TWO 8 AT 212 F.

Pounds Mod 1 t m Elo u i t:-

u us a nga on: n

mmin.at28lF i700 2050 40 min. It 281 F 2000 2075 K) min. at an" F 1050 iii!) 80 min. at our F r 1825 18(1) Tensile Strength I) min. at as) F A 2100 26(1) wmlmatwl"... 2175 wow I] min. at 280 F 2150 2125 Elongation at Break: Per cant I) min. at F 540 555 OmimatWF... 5% 5G) 81min. at can F- 520 520 RETAINED TENSILE STRENGTH Per cent M 68 Index 100 106 Nature of Cure Blank Test Material 40min. at F Whlte Very light cream. 00 min. at 287 F do White.

The above tests show that nonyiated cresylio 8 acid is an elective stabilizer in'natural rubber and substantially non-'discoloring.

The stabilizers of this invention are alkylated to an appreciable degree. The extent to which alkyl substituents are introduced into the cresylic acid will vary with the nature of the alkylation process employed, etc. In general. it may be said that for each molecule of the stabilizer there are, on the average, one to two alkyl substituents each comprising four to twelve carbon atoms.

By a study of hydroxyl numbers the number of alkyl substituents added per molecule of cresylic acid. on the average. by certain of the above processes, has been determined. The unalkylated acid contains no more than a trace of any substituent with tour or more carbon atoms. The number of such substituents added is, therefore. for all practical purposes, the amount of such substituents present.

Thus, the calculated hydroxyl numbers for monoand di-octylated cresylic acids average 7.23 and 4.01, respectively, whereas the calculated hydroxyl numbers of the starting cresylic acids themselves vary within the range of 12.5 to 15.7. The hydroxyl numbers for the products identifled as Examples 4, 4A, 4B and 46 were found to 7.47, 6.60, 8.30 and 4.75 respectively. The calculated numbers are based on approximations, and within the limits of experimental error it is shown that the octylated cresylic acid and its fraction average one to two octyl substituehts per molecule.

The calculated hydroxyl numbers for the monoand di-nonylated cresylic acids are 6.82 and 4.53. That found for the stabilizer of Example 5was 5.77.

The examples and tests are representative. The invention is not limited thereto. Other alkylated cresylic acids comprising for each molecule, one to two alkyl groups each containing four to twelve carbon atoms, and with an initial boiling point no lower than 100 C. at 10 mm. pressure, and derived from cresylic acids boiling at temperatures of 190 to 250 C. at atmospheric pressure are likewise good inhibitors and are included within the scope of this invention; and petroleum cresylic acids of any boiling range containing one to two such alkyl groups per molecule are stabilized and included in the invention. Any formula for sulfur vulcanization may be used. The stock need not be heavily loaded with white pigment. From 0.10 to 10 per cent of the inhibitors may be employed, and they may be used in coniunction with other stabilizers.

What I claim is:

1. Cured natural rubber which contains as flex-cracking inhibitor 9. small amount of a cresylic acid alkylated by introduction of one to two hydrocarbon substituents of four to twelve carbon atoms per cresylic acid molecule, the alkylated cresylic acid being constituted of a mixture of compounds which boils above C. at ten millimeters pressure and being derived from cresylic acid boiling at to 250 C. at atmospheric pressure.

2. Cured natural rubber which contains as flex-cracking inhibitor a small amount of a petroleum cresyllc acid alkylated by introduction of one to two hydrocarbon substituents of four to twelve carbon atoms per cresyilc acid molecule, the alkylated cresylic acid being constituted of a mixture of compounds which bolls above 100 C. at ten millimeters pressure and being derived from petroleum cresylic acid boiling at 190 to 250 C. at atmospheric pressure.

9 3. The process of curing natural rubber which comprises heating the same with sulfur and as a flex-cracking inhibitor a small amount oi a cresylic acid alkylated by introduction of one to two hydrocarbon substituents of tour to twelve carbon atoms per molecule 0! cresylic acid, the alkylated cresylic acid being constituted of a mixture 01 compounds which bolls above 100 C. at ten millimeters pressure and derived from cresylic acid boiling at 190 to 250 C. at atmospheric pressure. 7

4. The process of curing natural rubber which comprises heating the same with sulfur and as a fiexcracking inhibitor a small amount 01' a petroleum cresylic acid alkylated by introduction of one to two hydrocarbon substituents of tour to twelve carbon atoms per cresylic acid molecule. the alkylated cresylic acid being constituted of a mixture of compounds which boils above 100' C. at ten millimeters pressure and being derived from petrolemn cresylic acid boiling at 190 to 250 C. at atmospheric pressure.

5. Vulcanizable but unvulcanised rubber composition which contains as flex-cracking inhibitor 9. small amount of cresylic acid alkylated by in- 25 troductlon of one to two hydrocarbon substituents of iour to twelve carbon atoms per cresylic acid molecule. the alkylated cresylic acid beinz con- 10 stituted o! a mixture 0! compounds which boils above 100 C. at ten millimeters pressure and being derived from cresylic acid boiling at 190 to 250 C. at atmospheric pressure.

6. Vulcanizable but unvulcanized rubber composition which contains as flex-cracking inhibitor a small amount of petroleum cresylic acid alkylated by introduction of one to two hydrocarbon substituents 0! four to twelve carbon atoms per cresylic acid molecule, the alkylated cresylic acid being constituted of a mixture of compounds which bolls above 100 C. at ten millimeters pressure and being derived 1mm petroleum cresylic acid boiling at 190 to 250 C. at atmospheric pressure.

GEORGE E. P. sm'rn. Jl.

REFERENCES crrnn The following references are of record in the the of this patent:

UNITED STATES PATENTS Number Name Date 2,181,823 Stevens Nov. 28, 1039 2,334,470 Armstrong Nov. 16, 1943 2,445,735 Kitchen July 20, 1948 2,495,145 Smith et a1. Jan. 17, 1950 2,500,780 van Glider Mar. 14, 1050 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,581,906 J nuary 8, 1952 George E P. Smi 1h, Jr.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Let core Patent should read as corrected below.

Column 8, line 49, for "stabilized" reed stabilizers Signed and sealed this 29th day of October 1957.

KARL H. AXLINE ROBERT C WATSON Attesting Officer Ccmnisaioner of Patents 

1. CURED NATURAL RUBBER WHICH CONTAINS AS FLEX-CRACKING INHIBIT A SMALL AMOUNT OF A CREYSLIC ACID ALKYLATED NY INTRODUCTION OF ONE TO TWO HYDROCARBON SUBSTITUENTS OF FOUR TO TWELVE CARBON ATOMS PER CRESYLIC ACID MOLECULE, THE ALKYLATED CRESYLIC ACID BEING CONSTITUTED OF A MIXTURE IF COMPOUND WHICH BOILS ABOVE 100* C. AT TEN MILLIMETERS PRESSURE AND BEING DERIVED FROM CRESYLIC ACID BOILING AT 190* TO 250* C. AT ATMOSPHERIC PRESSURE. 